This invention relates to a method for the production of large single crystals of LiNdP.sub.4 O.sub.12 and BaB.sub.2 O.sub.4.
In the related application, Ser. No. 24,239, one of us and another member of our research team described a new process to grow substantially flaw-free large single crystals of a class of compounds by use of flux growth techniques, provided that the growth is carried out under essentially spatially isothermal conditions. This class of compounds has the formula MTiOXO.sub.4 where M is K, Tl, Rb or mixtures thereof, and X is P or As. Such crystals have various optical uses, as described in said application and the referenced publications. In the process described, the single crystals are grown from a melt produced by heating a mixture of MTiOXO.sub.4 and a flux comprising the oxides of M and X (wherein M and X have their above-designated meanings) in the ratio by weight of M to X of from 3 to 1 to 1 to 1, the ratio by weight of MTiOXO.sub.4 to flux at the highest heating temperature being substantially equal to the saturation value of the MTiOXO.sub.4 in the flux, or of their precursors, by slowly cooling the melt while the melt is maintained under essentially spatially isothermal conditions to thereby cause the MTiOXO.sub.4 to crystallize from the melt onto a seed crystal of MTiOXO.sub.4 suspended in the melt. The melt is considered to be maintained under essentially spatially isothermal conditions when the maximum difference in temperatures between any two points in the melt is not greater than about 4.degree. C., and preferably less than 2.degree. C. In a preferred embodiment, the defined spatially isothermal conditions at high-temperature used during the growth process are achieved by surrounding the crucible containing the melt with an elongated heat pipe, preferably constructed of a double-walled Inconel tube filled with sodium.
We have now discovered that the same kind of process can be employed to grow single crystals of other optically useful materials, specifically, lithium neodymium tetraphosphate (LiNdP.sub.4 O.sub.12) (also referred to as "LNP") and barium borate (BaBO.sub.2 O.sub.4) (also referred to as "BBO"). In the process of this invention, we use exactly the same furnace with elongated heat pipe as described in said related application, whose contents are hereby incorporated by reference, under essentially the same conditions, i.e., essentially spatially isothermal conditions (no temperature gradient as the driving force for the crystal growth) in which the maximum temperature difference between any two points in the melt is not greater than about 4.degree. C., preferably less than about 2.degree. C. We also prefer to suspend the seed approximately in the center of the melt, and to rotate the seed to ensure melt homogeneity. The ratio by weight of the crystal compound to the flux at the seeding temperature is substantially equal to the saturation value of the crystal compound in the said flux.
While the process of the present invention is particularly useful for crystals of LiNdP.sub.4 O.sub.12 and BaB.sub.2 O.sub.4, other related compounds can also be grown by this process, for example, with the Ba replaced by Sr, and/or the Nd replaced by any other rare earth element, and/or the Li replaced by K,Rb, or Cs. In addition, any flux which is liquid in the temperature range 500.degree.-1000.degree. C. can be utilized, e.g., 3K.sub.2 WO.sub.4. P.sub.2 O.sub.5, in addition to the preferred fluxes listed below.